1. Field of the Invention
This invention relates generally to marine seismic energy sources and more particularly to use of small explosive charges, fired by percussion, at short time intervals.
2. Description of the Prior Art
There are a number of known seismic exploration systems for launching small explosive charges into the water from a seismic exploration ship. Typically, a launcher assembly consists of a long, large-diameter flexible hose, one end of which is attached to a high volume water pump on the ship. To the other end of the hose is attached an accelerator barrel which is, in turn, coupled to a detonator device. The detonator device has rigidlymounted percussion element or firing pin on one end and an ejection window cut into one side between the firing pin and the acceleration barrel. The hose, acceleration barrel, and detonator device are towed beneath the water surface by the seismic ship at a convenient speed of four to six knots.
Explosive cartridges are provided. The cartridge consists of a can or a cardboard cylinder containing an explosive such as Nitro-Carbo-Nitrate (NCN) and a primer. A cylindrical well is provided at one end of the cartridge into which a rim-fire percussion cap, having a delay fuse, may be embedded to arm the charge.
In operation, the pump directs a high-volume stream of water through the attached launcher assembly. The armed cartridge is inserted into the water stream by means of a suitable loading device, cap-end first. The cartridge is flushed through the hose to the acceleration barrel whence it emerges into the detonator device at high velocity. The momentum of the cartridge causes the cap in the end of the cartridge to strike the firing pin, igniting the delay fuse. At the same time, the force of the water stream nudges the trailing end of the cartridge out of the lateral ejection window of the detonator device, end-over-end. The cap delay is one or two seconds to provide sufficient time for the ship to tow the launcher a safe distance away from the exploding cartridge when the delay fuse times out.
A description of the cartridge and delay fuse is disclosed in U.S. Pat. No. 3,574,298 in conjunction with FIGS. 2-2d of the patent. Several versions of a detonator device or "gun" are shown FIGS. 1, 3, 6, and 7. Improved versions of the same gun are disclosed in U.S. Pat. No. 3,601,052 FIGS. 1-5 and U.S. Pat. No. 3,968,855, FIG. 2. FIG. 2 of the latter patent also illustrates the details of an acceleration barrel. FIGS. 3 and 4 of U.S. Pat. No. 3,601,052 disclose an improved means for nudging the cartridge out of the ejection window of the gun.
Basically, in all of the commercially successful prior art devices discussed, the cartridge strikes a rigid firing pin and is ejected sideways from the detonator device. Small charges on the order of a half-pound of explosive are conveniently enclosed in small metal cartridges which have sufficient rigidity and a small enough mass so that they are not deformed when the cartridge strikes the rigid firing pin. Larger charges, of 6 to 8 pounds, are most conveniently enclosed in elongated cardboard tubes. Because of the considerably larger mass, the cardboard-enclosed charges disintegrate when they abruptly impact the rigid firing pin. Premature detonations result, destroying the launcher assembly.
The second disadvantage of the above-cited devices is the sideways ejection of the cartridges. It is not unusual for the charge to remain unejected with consequent premature detonation. Known causes for such non-ejection are a drop in pressure of the cartridge-flushing water stream and entanglement of the gun ejection window by underwater debris, weeds or the like. It would be preferable to linearly eject the cartridge from an open-ended detonator device and to provide a rotatable firing pin assembly which would remove itself from the pathway of the cartridge after the delay fuse has been ignited.
FIGS. 6-6b of U.S. Pat. No. 3,574,298 illustrate one such embodiment. A spring loaded firing pin assembly 86 is mounted on the open end of gun 84. When the cap in the forward end of cartridge 4' strikes knife edge 98, the delay fuse is ignited. The knife edge thereafter moves upwards and slides across the side of cartridge 4' which is supposed to be ejected linearly from the open end of the gun. The device is unsatisfactory for three reasons. First, in commercial use, the firing pin assembly 86 was found to have an insufficient moment of inertia to reliably ignite the cap. As is well known, a substantial impactive force is required to detonate a rim-fire percussion cap. Second, the edge 98 of the blade 86' under the urging of the spring 96 exerts pressure on and cuts the side of the casing of cartridge 4', thereby disabling the charge. Third, the downward force of the spring-loaded blade against the side of the cartridge tends to misalign the cartridge with respect to the logitudinal axis of the gun 84 as the cartridge emerges. Hangups occur, resulting in premature detonation and destruction of the gun.